1. Field of the Invention
The present invention relates to hearing protection devices. More particularly, the present invention relates to a system and method for testing hearing protection devices in the field to determine their effectiveness as actually used, not as theoretically used.
2. General Background of the Invention
Approximately nine million workers in the United States are exposed to potentially hazardous noise levels in the workplace.
Federal regulation of occupational noise exposure levels (OSHA: CFR 1910.95) has been in place for almost three decades. That regulation provides for use of hearing protection devices in numerous instances of exposure to potentially hazardous levels of workplace noise in order to prevent occupational noise-induced hearing loss. Manufacturers of hearing protection devices are required to label products in terms of noise reduction capability according to the procedures promulgated by the Environmental Protection Agency. That specification is designated as the Noise Reduction Rating (NRR). The method used to calculate the NRR is a laboratory procedure. For more than ten years, scientific research has extensively and consistently demonstrated that the NRR in no way accurately represents the actual, "real world" attenuation of workplace noise which is provided by hearing protection devices. Numerous variables, such as individual fit, insertion or application technique, training, etc., affect the "real world" attenuation provided by hearing protectors. As a result, several computational procedures have been proposed in the scientific literature (such as derating the NRR by 7 dB or by 50%) in an attempt to render a modification of the NRR a more realistic estimate of the actual protection provided. However, all such techniques remain computational, and do not provide a means of objectively measuring the actual level of workplace noise which penetrates the hearing protection device and reaches the worker's hearing mechanism, possibly causing irreversible hearing loss.
U.S. Pat. No. 3,729,598 to Tegt et al. describes a system for measuring the effectiveness of the sound attenuating function of an earphone sound attenuation cover by employing the earphone transducer as a microphone and measuring the noise level with the cover on and with the cover removed, the difference being interpreted as the acoustic attenuation provided by the ear cover.
U.S. Pat. No. 4,020,298 to Epley et al. describes a portable device primarily for use in determining the effectiveness of various personal hearing protectors in attenuating hazardous noise. This is a subjective method whereby hearing tests are conducted using either threshold data in quiet or supra-threshold data in the presence of noise with the protection in place and compared to the open ear (protector out of ear) to establish an audiometric test differential corresponding to the level of protector performance.
U.S. Pat. No. 3,968,334 to Padilla describes a subjective method and apparatus for audiometrically testing the effectiveness and fitting of insert-type hearing protective devices. Each ear is enclosed under an earmuff which allows the pinna to be free of physical contact so as not to distort the ear canal in which the ear plug is inserted. The muff is equipped with an earphone to transmit audiometric signals. The series of transmitted signals are repeated both without and with ear plugs installed in the ears to obtain the relative difference between the two hearing thresholds.
U.S. Pat. No. 5,044,373 to Northeved et al. describes a method of measuring sound pressure levels in the auditory canal of a person in connection with the fitting of a hearing aid. The invention requires two microphones, one for calibration and one connected to a probe tube of known acoustic characteristics. An objective differentiation between the calibration microphone and the probe microphone yield an end result equal to the respective gain or attenuation of the hearing aid. Neither microphone is on the medial surface of the hearing aid and further more precise tube placement, relative to the distance from the ear drum to the tube tip, is necessary to ensure measurements are reproducible from test to test on the same subject by different technicians.
U.S. Pat. No. 5,317,273 discloses a hearing protection device evaluation apparatus for testing the effectiveness of muff-type hearing protectors. It mentions at column 1, fines 26-31 a method of testing ear plugs using an artificial head in which one microphone is placed "inside the head near the artificial ear drum" and the other is placed outside the muff. U.S. Pat. No. 4,586,194 discloses such an artificial head being used to measure earphones.
U.S. Pat. No. 4,060,701 is similar to U.S. Pat. No. 4,020,298 but only measures effectiveness subjectively.
Other patents which may be of interest include the following:
______________________________________ U.S. Pat. No. 3,697,973 to Stevens et al. U.S. Pat. No. 3,757,769 to Arguimbau et al. U.S. Pat. No. 3,906,158 U.S. Pat. No. 4,024,499 U.S. Pat. No. 4,029,083 to Baylor U.S. Pat. No. 4,064,362 U.S. Pat. No. 4,416,155 U.S. Pat. No. 4,586,194 to Kohashi et al. U.S. Pat. No. 4,763,753 to Killion U.S. Pat. No. 4,781,196 to Killion U.S. Pat. No. 4,813,430 to Hecox et al. U.S. Pat. No. 4,852,683 to Killion U.S. Pat. No. 4,966,160 to Birck et al. U.S. Pat. No. 5,113,967 to Killion et al. ______________________________________
Whether the failure of a hearing protection device to adequately protect a worker is due to a defective device or improper placement of a hearing protection device in the worker's ear, the potential result can be the same--otherwise preventable noise-induced hearing loss.